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                [Sdoi2014]重建                </h1>
                <p>时间限制：10s&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;  空间限制：512MB</p>			</div>
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                                <h3>题目描述</h3><p><p><span style="font-size: medium">&nbsp; T国有N个城市，用若干双向道路连接。一对城市之间至多存在一条道路。<br />
&nbsp;&nbsp;&nbsp; 在一次洪水之后，一些道路受损无法通行。虽然已经有人开始调查道路的损毁情况，但直到现在几乎没有消息传回。<br />
&nbsp;&nbsp;&nbsp; 辛运的是，此前T国政府调查过每条道路的强度，现在他们希望只利用这些信息估计灾情。具体地，给定每条道路在洪水后仍能通行的概率，请计算仍能通行的道路恰有N-1条，且能联通所有城市的概率。<br />
</span></p></p><hr/><h3>输入格式</h3><p><p><font size="4">&nbsp; 输入的第一行包含整数N。<br />
&nbsp; 接下来N行，每行N个实数，第i+l行，列的数G[i][j]表示城市i与j之<br />
间仍有道路联通的概率。<br />
&nbsp;&nbsp;&nbsp; 输入保证G[i][j]=G[j][i]，且G[i][j]=0；G[i][j]至多包含两位小数。<br />
</font></p></p><hr/><h3>输出格式</h3><p><p><br />
<font size="4">&nbsp;&nbsp;&nbsp; 输出一个任意位数的实数表示答案。<br />
&nbsp;&nbsp;&nbsp; 你的答案与标准答案相对误差不超过10^(-4)即视为正确。<br />
<br />
</font></p></p><hr/><h3>样例输入</h3><pre>
    3
    0 0.5 0.5
    0.5 0 0.5
    0.5 0.5 0
</pre><hr/><h3>样例输出</h3><pre>    0.375</pre><hr/><h3>提示</h3><p><p><span style="font-size: medium">1 &lt; N &lt; =50</span></p>
<p><span style="font-size: medium">数据保证答案非零时，答案不小于10^-4</span></p></p><hr/><h3>题目来源</h3><p>Round 1 Day 2</p>
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